microbit: BUGFIX - correct initialization of MicroBitListener

code now correctly initiailizes the evt_queue filed needed to prevent
lockup during event processing (understandably!).

source/MicroBitListener.cpp

@@ -23,6 +23,7 @@ MicroBitListener::MicroBitListener(uint16_t id, uint16_t value, void (*handler)(
 	this->cb_arg = NULL;
     this->flags = flags;
 	this->next = NULL;
+    this->evt_queue = NULL;
 }
 
 /**
@@ -41,6 +42,7 @@ MicroBitListener::MicroBitListener(uint16_t id, uint16_t value, void (*handler)(
 	this->cb_arg = arg;
     this->flags = flags | MESSAGE_BUS_LISTENER_PARAMETERISED;
 	this->next = NULL;
+    this->evt_queue = NULL;
 }
 
 /**

source/MicroBitMessageBus.cpp

@@ -66,7 +66,7 @@ void async_callback(void *param)
         else
             listener->cb(listener->evt);
 
-        // If there are more events to process, dequeue te next one and process it.
+        // If there are more events to process, dequeue the next one and process it.
         if ((listener->flags & MESSAGE_BUS_LISTENER_QUEUE_IF_BUSY) && listener->evt_queue)
         {
             MicroBitEventQueueItem *item = listener->evt_queue;